1. Field of the Invention
The present invention relates to a process for producing a lithium-cobalt composite oxide for a positive electrode of a lithium secondary battery, which has a large volume capacity density, high large current discharge properties, high safety, high charge and discharge cyclic durability, high press density and high productivity, by using cobalt hydroxide and lithium carbonate as inexpensive materials, a positive electrode for a lithium secondary battery containing the produced lithium-cobalt composite oxide, and a lithium secondary battery.
2. Discussion of Background
Recently, as the portability and cordless tendency of instruments have progressed, a demand for a non-aqueous electrolyte secondary battery such as a lithium secondary battery which is small in size and light in weight and has a high energy density, has been increasingly high. As a positive electrode active material for the non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2, LiMn2O4 or LiMnO2, has been known.
Among them, a lithium secondary battery using a lithium-cobalt composite oxide (LiCoO2) as a positive electrode active material and using a lithium alloy or carbon such as graphite or carbon fiber as a negative electrode, can obtain a high voltage at a level of 4V, whereby it has been widely used as a battery having a high energy density.
As a method for producing a lithium-cobalt composite oxide, it is common to employ tricobalt tetroxide as the cobalt material and lithium carbonate as the lithium material. Further, a method of employing cobalt oxyhydroxide and lithium carbonate has been recently industrially employed. Such tricobalt tetroxide or cobalt oxyhydroxide is produced by oxidation of cobalt hydroxide. Cobalt hydroxide is inexpensive as a cobalt material, since it is the raw material for tricobalt tetroxide or cobalt oxyhydroxide.
JP-2002-321921 discloses a method for producing lithium cobaltate from cobalt hydroxide and lithium carbonate. In this JP-A-2002-321921, it is characterized that firstly a cobalt hydroxide powder and a lithium carbonate powder are mixed so that the molar ratio of Li/Co will be from 1.02 to 1.06, the mixture is granulated and subjected to primary firing at a temperature of from 600 to 700° C., and then, the fired product is pulverized, followed further by secondary firing at a temperature of from 750 to 1,000° C. However, the method disclosed in JP-A-2002-321921 includes steps of granulation of a raw material mixed powder prior to firing and pulverization after the primary firing and thus has a problem that the production process tends to be cumbersome, the production cost tends to be high, or the charge and discharge cyclic durability tends to be poor. Accordingly, it is unexpected even to those skilled in the art that it is possible to produce a lithium-cobalt composite oxide which has a particle size distribution desirable for a positive electrode of a lithium secondary battery and which will become a positive electrode having good performance, by using cobalt hydroxide as an inexpensive material.